Doppler-Broadened Iron X-ray Lines from Tycho's Supernova Remnant

We use \suzaku observations to measure the spatial variation of the Fe K$\alpha$ line with radius in the \tycho supernova remnant. The Fe line widths show a significant decrease from a FWHM value of 210 eV at the center to 130 eV at the rim. Over the same radial range the line center energy remains nearly constant. These observations are consistent with a scenario in which the shell of Fe-emitting ejecta in \tycho is expanding at speeds of 2800--3350 km s$^{-1}$. The minimum line width we measure is still a factor of two larger than expected from a single component plasma emission model. If thermal Doppler broadening is the dominant additional source of broadening, we infer an ion temperature of $(1--3) \times 10^{10}$ K.